In recent years, as a virtual image display apparatus such as a head-mounted display capable of forming and observing a virtual image, various apparatuses that guide image light rays from a display apparatus to the pupil of an observer by using a light guide plate have been suggested.
For example, as a device that guides a collimated image and the like to an observer's angle of view, a device configured such that a plurality of half mirrors (hereinafter, referred to as “HMs”) is arranged in a parallel planar light guide plate and image light rays are reflected from the HMs so as to be presented to the observer has been known (see PTLs 1 to 5).
In an apparatus of PTL 1, image light rays guided from one end of a light guide plate are reflected while successively transmitting a plurality of HMs that obliquely traverses the light guide plate, and reach an observer. In an apparatus of PTL 2, image light rays guided to the light guide plate propagate while being totally reflected in the light guide plate, are reflected from an outer surface, are reflected from a plurality of HMs that obliquely traverses the light guide plate, and ultimately head for an observer. In an apparatus of PTL 3, image light rays guided to the light guide plate propagate while being totally reflected in the light guide plate, are reflected from a surface close to an observer, are reflected from a plurality of HMs that obliquely traverses the light guide plate, and ultimately head for the observer. In this case, in order to increase efficiency, among the light rays incident on the HMs, the reflectance of the light rays having a large incident angle (50 degrees to 70 degrees) is set to be approximately 0, and the reflectance of the light rays having a small incident angle (40 degrees or less) is set to be a predetermined reflectance. In an apparatus of PTL 4, image light rays guided to a light guide plate propagate while being totally reflected in the light guide plate, are reflected from a surface close to an observer, are reflected from a plurality of HMs, and ultimately head for the observer. Here, the thickness of a region or a layer in which the HMs are provided is set to be thinner than the light guide plate, and thus, the observer can observe the image light rays without transmitting the image light rays through the HMs. In an apparatus of PTL 5 shown in FIG. 1, image light rays guided to a light guide plate propagate while being totally reflected in the light guide plate, are reflected from a surface close to the external side, are reflected from the HMs, and ultimately head for an observer. Here, a region in which the HMs are provided has the same thickness as the light guide plate, and the reflectance of the HMs is gradually increased as the HM becomes farther from a display.
As another display apparatus, there is an apparatus including a thin micro-mirror array adhering to one side of a parallel plate type light guide plate (see PTL 6). In this apparatus, in order to form an image, a scanning beam image source is used, and the pupil is also vertically expanded. Scanning light rays guided to the light guide propagate in the light guide plate and a half mirror array, are reflected from a surface close to the external side, are reflected from HMs of the half mirror array close to an observer, and ultimately head for the observer.
In the apparatuses described in PTLs 1 to 3 and 5, since luminance is reduced whenever the image light rays transmit through the HMs, non-uniformity occurs in the view of angle, and it is not easy to solve or suppress such non-uniformity. In order to solve the luminance non-uniformity, when the reflectance of the HM present on an internal side or a side opposite to a light source is gradually increased, the transmittance of the HM is decreased according to the increase in the reflectance thereof, and non-uniformity occurs in external light (see-through light).
In the apparatus described in PTL 4, since the thickness of a reflection unit configured such that the HMs are arranged is thinner than a light guide, the light rays reach the HM on the internal side without transmitting other HMs. For this reason, non-uniformity in light amount does not occur. However, a HM section configured such that two small surfaces are paired is disposed in the reflection unit, and the image light rays are reflected from the HM twice. For this reason, reflection efficiency tends to be decreased.
In the apparatus described in PTL 6, in order to widen the width of a pencil of light rays, the pencil of light rays is divided using the HMs. In so doing, as the image light rays propagate toward the internal side, luminance is decreased, and non-uniformity occurs in the angle of view. Thus, it is not easy to solve or suppress such non-uniformity.